High entropy oxides

Abstract

High entropy oxides and methods of making the same are disclosed. The high entropy oxide is characterized by a submicron particle size and a rod-like particle shape. The preparation method comprises a coprecipitation step, and an oxalate compound is preferably used as a precipitator. Also disclosed are electrodes, such as anodes, catalysts and electrochemical cells, comprising the high entropy oxides.

Description

Technical Field

[0001] The present disclosure relates to high entropy oxides, methods for preparing the same, and their use as electrode materials and catalysts. Background Art

[0002] High entropy oxides, also known as entropy-stabilized oxides, have received much attention due to their outstanding compositional and structural stability under extreme conditions such as extreme temperature and chemical environments. In addition, many other attractive and unique properties have been found in these materials, such as superior superionic conductivity at room temperature, high dielectric constants, and tailorable band gaps. Therefore, high entropy oxides can be used as anode materials (e.g., in lithium batteries), cathode materials, and catalysts.

[0003] Solid-state synthesis is the most common and easy method to make high entropy oxides. For example, Qiu et al. (Qiu, N.; Chen, H.; Yang, Z.; Sun, S.; Wang, Y.; Cui, Y., “A high entropy oxide (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 High entropy oxide (Mg O) with superior lithium storage performance 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O)" Journal of Alloys and Compounds [Journal of Alloys and Compounds] 2019, 777, 767-774) describes a method in which MgO, CoO, NiO, CuO and ZnO are mixed in a planetary ball mill, then pressed into pellets and sintered at 1000°C for 24 hours. However, the particles synthesized by solid-state methods are generally large, which adversely affects the application of entropy-stabilized oxides. For example, due to the limited specific surface area of ​​the catalyst, large particles tend to slow down the catalytic reaction rate. A recently developed variant of the solid-state method uses atomized spray pyrolysis to synthesize (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2)O (Sarkar, A.; Velasco, L.; Wang, D.; Wang, Q.; Talasila, G.; deBiasi, L.; Kübel, C.; Brezesinski, T.; Bhattacharya, SS; Hahn, H., "High entropy oxides for reversible energy storage," Nature communications 2018, 9(1), 1-9). The entropy-stabilized oxide particles synthesized by this method were found to be hollow or solid spheres with particle sizes ranging from nanometers to micrometers. Such a wide size distribution may lead to high overpotentials on large particles when these particles are used as electrode materials.

[0004] Alternatively, coprecipitation methods can be used to synthesize nanosized high entropy oxides. These methods generally involve converting metal cations into hydroxide precursors, and then annealing the precursors to provide oxide products. Sodium hydroxide and ammonia solutions are commonly used to prepare hydroxide precursors. However, these methods create many practical problems. For example, because sodium hydroxide and ammonia solutions react rapidly with metal cations, it is difficult to adjust the shape and size distribution of the synthesized particles. In addition to direct hydroxide sources such as NaOH, hexamethylenetetramine (HMTA) or urea are sometimes used as precipitants for uniform deposition. In these cases, the functional component for precipitation is ammonia produced by thermal decomposition of the precipitant, which is then dissolved in water to produce ammonium hydroxide. When a stoichiometric amount of ammonia is added, low alkalinity results in Mg 2+ On the other hand, excessive addition of urea or HMTA will lead to the precipitation of Cu(OH) 2 After coprecipitation, the obtained precursor usually undergoes a subsequent annealing process at high temperature. Such annealing leads to severe agglomeration of ultrafine high entropy oxide particles. Therefore, the synthesis of high entropy oxides with submicron particles of narrow size distribution and uniform composition remains challenging.

[0005] Purpose

[0006] It is therefore an object of the present invention to obviate to some extent the above disadvantages; and / or at least to provide the public with a useful choice.

[0007] Other objects of the present invention will become apparent from the following description which is given by way of example only. Summary of the invention

[0008] definition

[0009] "Annealing process" and "solutionizing process" refer to heat treatment of a material that allows a certain degree of atomic migration in order to reduce structural defects in the material.

[0010] "Calcination process" means a thermal treatment of a material wherein organic components of the material are removed, for example by oxidation or gasification.

[0011] "High entropy oxide" means an oxide material characterized by the presence of four or more elementally different metal cations within the ionic structure.

[0012] Oxide materials comprising four elementally different metal cations may be referred to in the art as "medium entropy oxides." For the avoidance of doubt, in this application, the term "high entropy oxide" encompasses oxide materials characterized by the presence of four or more elementally different metal cations within the ionic structure.

[0013] The four or more different metal cations on the elements are preferably present in the high entropy oxide in substantially equimolar amounts, although this is not required. For example, each metal cation may account for at least 5% of the total number of different metal cations on the four or more elements in the high entropy oxide.

[0014] "Metal cations" include cations of metal elements of Group 2 (alkaline earth metals) and Groups 3-12 (transition metals).

[0015] "Particle shape" means the shape of particles. Particle shapes include spheres, rods, cubes and plates.

[0016] "Granularity D 50 ” means the median particle size of the sample.

[0017] "Granularity D 90 ” means the 90th percentile particle size of the sample.

[0018] In a first aspect, the present invention relates to a method for preparing a high entropy oxide, the method comprising:

[0019] a. providing at least four metal salts;

[0020] b. mixing the metal salts in a solvent to form a solution;

[0021] c. mixing the solution obtained in step (b) with a precipitant to obtain a precipitate;

[0022] d. heat treating the precipitate of step (c) to obtain a high entropy oxide.

[0023] In some embodiments, the heat treatment step (d) comprises heating the precipitate for a period of at least about 30 minutes. In some embodiments, the heat treatment comprises heating the precipitate for a period of about 1 hour, about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, or about 8 hours.

[0024] In some embodiments, the heat treatment of step (d) includes a calcination process. In some embodiments, the calcination process is carried out at a temperature of about 300°C to about 1200°C. In some embodiments, the calcination process is carried out at a temperature of about 350°C to about 450°C. In some embodiments, the calcination process is carried out at a temperature of about 400°C. In some embodiments, the calcination process is carried out at a temperature of about 900°C to about 1100°C. In some embodiments, the calcination process is carried out at a temperature of about 1000°C.

[0025] In some embodiments, the heat treatment step (d) comprises a calcination process to provide an oxide intermediate. In some embodiments, the calcination process comprises heating the precipitate at a temperature of about 300° C. to about 500° C. for a period of about 2 hours to about 4 hours to form the oxide intermediate.

[0026] In some embodiments, the heat treatment step (d) comprises an annealing process. In some embodiments, the annealing process comprises heating the oxide intermediate at a temperature of about 900° C. to about 1100° C. for a period of about 3 hours to about 6 hours. In some embodiments, the annealing process comprises heating the oxide intermediate for a period of at least 5 hours.

[0027] In some embodiments, the heat treatment step (d) includes an annealing process immediately after the calcining process, for example in the same apparatus, without isolating the oxide intermediate. In some embodiments, the oxide intermediate is cooled (for example, cooled to room temperature) prior to the annealing process.

[0028] In some embodiments, the heat treating step (d) comprises a combined calcining and annealing process comprising heating the precipitate at a temperature of about 900° C. to about 1100° C. for a period of about 3 hours to about 6 hours.

[0029] In some embodiments, the oxide intermediate is mixed with a solid dispersant prior to annealing. In some of these embodiments, the oxide intermediate is suspended in a solvent (eg, water) with a solid dispersant, and the suspension is then dried and the resulting powder is ground prior to annealing.

[0030] In some embodiments, the amount of the solid dispersant is greater than about 5 times (by weight) the amount of the oxide intermediate. In some embodiments, the amount of the solid dispersant is greater than about 10 times (by weight) the amount of the oxide intermediate. In some embodiments, the amount of the solid dispersant is about 5 to about 15 times (by weight) the amount of the oxide intermediate. In some embodiments, the amount of the solid dispersant is about 10 times (by weight) the amount of the oxide intermediate.

[0031] In some embodiments, the solid dispersant is a material having a melting point higher than the annealing temperature. In some embodiments, the solid dispersant is water-soluble. In some embodiments, the solid dispersant is inert. Preferably, the solid dispersant is selected from the group consisting of potassium sulfate, potassium phosphate, sodium phosphate, sodium aluminate, and any combination of two or more thereof. In some embodiments, the solid dispersant is potassium sulfate.

[0032] In some embodiments, the method further comprises washing the product obtained from the annealing process with the solid dispersant with water to remove the solid dispersant.

[0033] In some embodiments, the solid dispersant is selected from the group consisting of potassium sulfate, potassium phosphate, sodium phosphate, sodium aluminate, and any combination of two or more thereof. In some embodiments, the solid dispersant is potassium sulfate.

[0034] In some embodiments, the heat treatment step (d) comprises using a controlled atmosphere. In some embodiments, the controlled atmosphere comprises oxygen.

[0035] In some embodiments, four metal salts are provided in step (a). In some embodiments, five metal salts are provided. In some embodiments, six or more metal salts are provided.

[0036] In some embodiments, the metal salts are selected from the group consisting of salts of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb, and Pt. In some embodiments, the metal salts are each independently selected from chloride salts, nitrates, sulfates, or acetates.

[0037] In some embodiments, the metal salts are salts of Mg, Ni, Cu, Co, and Zn. In some embodiments, the Mg salt, Co salt, Ni salt, and Cu salt are each independently selected from chloride salts, nitrates, sulfates, or acetates. In some embodiments, the Mg salt, Co salt, Ni salt, and Cu salt are chloride salts. In some embodiments, the Zn salt is a nitrate, sulfate, or acetate. In some embodiments, the Zn salt is a nitrate.

[0038] In some embodiments, the metal salt is a salt of Mg, Mn, Fe, Co and Ni. In some embodiments, the Mg salt, Co salt, Ni salt and Cu salt are each independently selected from chloride salts, nitrates, sulfates or acetates. In some embodiments, the Mg, Mn, Fe, Co and Ni salts are chloride salts.

[0039] In some embodiments, the precipitant is a hydroxide compound (e.g., NaOH), an organic compound (e.g., an oxalate compound), or an ammonia source (e.g., hexamethylenetetramine or urea). A preferred precipitant is an oxalate compound. In some embodiments, the oxalate compound is soluble in a solution of water and ethylene glycol, such as a 1:2 water:ethylene glycol solution (by volume). In some embodiments, the oxalate compound is ammonium oxalate. In some embodiments, the oxalate compound is ammonium oxalate monohydrate.

[0040] In some embodiments, the solvent used to form the metal salt solution in step (b) is water, such as deionized water; water and an organic solvent; or an organic solvent. Preferred solvents include water and ethylene glycol. In some embodiments, the solvent includes a mixture of water and ethylene glycol, wherein the ratio of water to ethylene glycol is in the range of 1.5:2 to 0.5:2 (by volume). In some embodiments, the solvent includes water and ethylene glycol in a ratio of about 1:2 (by volume).

[0041] The different metal cations on the four or more elements are preferably present in substantially equimolar amounts, although this is not necessary. In some embodiments, the amount of each of the metal cations is at least about 5% of the total amount of metal cations. Each metal cation can account for at least 10%, at least 15%, at least 20%, or at least 25% of the total amount of metal cations in the high entropy oxide. Each metal cation in the high entropy oxide can account for 5% to 30% of the total amount of metal cations in the high entropy oxide. For example, each metal cation in the high entropy oxide can account for 10% to 30% of the total amount of metal cations, or 15% to 30%, or 20% to 30%.

[0042] In some embodiments, the precipitant is dissolved in a solvent before mixing with the solution obtained in step (b). In some embodiments, the solvent for dissolving the precipitant is water. In some embodiments, the solvent comprises a mixture of water and an organic solvent such as ethylene glycol. In some embodiments, the solvent comprises a mixture of water and ethylene glycol, wherein the ratio of water to ethylene glycol is in the range of 1.5:2 to 0.5:2 (by volume). In some embodiments, the solvent comprises water and ethylene glycol in a ratio of about 1:2 (by volume).

[0043] In some embodiments, the precipitate in step (c) is obtained in a solvent comprising water, such as deionized water; water and an organic solvent; or an organic solvent. In some embodiments, the solvent comprises a mixture of water and ethylene glycol in a ratio ranging from 1.5:2 to 0.5:2 (by volume). In some embodiments, the solvent comprises water and ethylene glycol in a ratio of about 1:2 (by volume).

[0044] In some embodiments, the high entropy oxide has an entropy-stabilized crystalline structure.

[0045] In another aspect, the present invention relates to a high entropy oxide prepared by the method of the present invention.

[0046] On the other hand, the present invention relates to a method for controlling the dispersibility of the particle size of a high entropy oxide, wherein the above-mentioned method for preparing a high entropy oxide is used, and the heat treatment of step (d) is carried out at a controlled temperature and / or time to produce the desired particle size dispersibility.

[0047] In another aspect, the present invention provides a v B w C x D y E z )O, wherein v, w, x, y and z are each independently from about 0.05 to about 0.30, provided that v+w+x+y+z=1; each of A, B, C, D and E is a different element selected from the list consisting of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb and Pt; and wherein the high entropy oxide is characterized by any one or more of the following:

[0048] The X-ray powder diffraction spectrum includes (111) and (200) peaks, among which I (111) / I (200) The relative peak intensity is greater than about 1;

[0049] Average particle size D 50 is about 800 nm or less;

[0050] ·Particle size D 90 is about 1200 nm or less;

[0051] Submicron particles have a rod-like shape;

[0052] • The particle size distribution has a standard deviation of less than about 250 nm.

[0053] In some embodiments, v, w, x, y, and z are each about 0.2.

[0054] In some embodiments, the high entropy oxide is represented by the formula (Mg v Co w Ni x Cu y Zn z )O or formula (Mg v Mn w Fe x Co y Ni z )O indicates.

[0055] In some embodiments, I (111) / I (200) The relative peak intensity of is about 1.2 to about 2.0, about 1.3 to about 1.9, about 1.4 to about 1.8, or about 1.5 to about 1.7. Preferably, I (111) / I (200) The relative peak intensity is about 1.6.

[0056] In some embodiments, the high entropy oxide is characterized by an X-ray powder diffraction spectrum comprising (111), (200), and (220) peaks. (220) / I (200) The relative peak intensity of is from about 0.3 to about 1.1, from about 0.4 to about 1.0, from about 0.5 to about 0.9, or from about 0.6 to about 0.8. Preferably, I (20) / I (200) The relative peak intensity is 0.7.

[0057] In some embodiments, the high entropy oxide is characterized by an X-ray powder diffraction spectrum comprising peaks at (111), (200), (220), (311), and (222). (311) / I (222) The relative peak intensity is less than about 1.

[0058] In some embodiments, the high entropy oxide is characterized by an X-ray powder diffraction spectrum substantially as Figure 5 shown.

[0059] In some embodiments, the high entropy oxide is characterized by having a substantially uniform particle size distribution.

[0060] In some embodiments, the high entropy oxide has a particle size distribution with a standard deviation of less than about 245 nm, less than about 240 nm, less than about 235 nm, less than about 230 nm, less than about 225 nm, less than about 220 nm, less than about 215 nm, less than about 210 nm, less than about 205 nm, or less than about 200 nm. In some embodiments, the high entropy oxide has a particle size distribution with a standard deviation of about 215 nm and a relative standard deviation of about 0.34.

[0061] In some embodiments, the high entropy oxide is characterized by an average particle size D 50 is about 700 nm or less. In some embodiments, the high entropy oxide is characterized by an average particle size D 50 is about 600 nm or less. In some embodiments, the high entropy oxide is characterized by an average particle size D 50 In some embodiments, the high entropy oxide is characterized by a particle size D 90 is about 1100 nm or less. In some embodiments, the high entropy oxide is characterized by a particle size D 90 is about 1000 nm or less. In some embodiments, the high entropy oxide is characterized by a particle size D 90 It is about 1000nm.

[0062] In some embodiments, the length: width ratio of the submicron particles having a rod-like shape is about 1: 1.5 to about 1: 3.5. In some embodiments, the length: width ratio of the submicron particles is about 1: 2 to about 1: 3. In some embodiments, the length: width ratio of the submicron particles is about 1: 2.5. In some embodiments, the average length of the submicron particles is about 400nm to about 1000nm. In some embodiments, the average length of the submicron particles is about 500nm to about 900nm. In some embodiments, the average length of the submicron particles is about 630nm. In some embodiments, the average width of the submicron particles is about 150nm to about 450nm, about 200nm to about 400nm, or about 250nm to about 350nm. In some embodiments, the average width of the submicron particles is about 300nm.

[0063] In some embodiments, the high entropy oxide is substantially non-porous. In some embodiments, the high entropy oxide is non-porous.

[0064] In some embodiments, the high entropy oxide is substantially homogeneous. In some embodiments, the high entropy oxide is a single phase.

[0065] In some embodiments, the high entropy oxide has a lattice parameter of at least about Preferably about to about between, more preferably about to about In some embodiments, the lattice parameter is about Preferably, the lattice parameter is

[0066] In yet another aspect, the present invention relates to an electrode, such as an anode or a cathode, comprising a high entropy oxide according to the present invention.

[0067] In some embodiments, the electrode comprises at least about 70% (by weight) high entropy oxide. In some embodiments, the electrode comprises at least about 80% (by weight) high entropy oxide. In some embodiments, the electrode comprises about 80% (by weight) high entropy oxide.

[0068] In some embodiments, the electrode is an anode. In some embodiments, the anode further comprises a conductive additive such as carbon black (e.g., Super P carbon black), acetylene black, conductive graphite, Ketjen black TM , carbon nanotubes, or a combination of any two or more thereof. In some embodiments, the anode comprises about 5% to about 15% (by weight) of a conductive additive. In some embodiments, the anode comprises about 10% (by weight) of a conductive additive. In some embodiments, the anode further comprises a binder, such as polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), or a combination thereof. In some embodiments, the anode comprises about 5% to about 15% (by weight) of a binder. In some embodiments, the anode comprises about 10% (by weight) of a binder. In some embodiments, the anode further comprises another anode material.

[0069] In some embodiments, the anode has a specific capacity of at least about 600mAh / g. In some embodiments, the anode has a specific capacity of about 600mAh / g to about 1200mAh / g. In some embodiments, the anode has a specific capacity of at least about 700mAh / g, about 800mAh / g, about 900mAh / g, about 100mAh / g, about 1100mAh / g or about 1200mAh / g. In some embodiments, the anode has a specific capacity of about 800mAh / g at a current rate of 0.2A / g. In some embodiments, the anode has a specific capacity of at least about 800mAh / g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 900mAh / g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 1000mAh / g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 1100mAh / g after 400 cycles. In some embodiments, the anode has a specific capacity of at least about 800mAh / g after 470 cycles. In some embodiments, the anode has a specific capacity of at least about 900 mAh / g after 470 cycles. In some embodiments, the anode has a specific capacity of at least about 1000 mAh / g after 470 cycles. In some embodiments, the anode has a specific capacity of at least about 1100 mAh / g after 470 cycles.

[0070] In yet another aspect, the present invention relates to a catalyst comprising a high entropy oxide according to the present invention. In some embodiments, the catalyst can be used to catalyze a process selected from water gas shift, steam reforming, Fischer-Tropsch synthesis or CO 2 Reduction reaction to synthesize groups consisting of higher alcohols.

[0071] In another aspect, the invention relates to an electrochemical cell comprising an anode, a cathode, a separator between the anode and the cathode, and an electrolyte, wherein the anode comprises a high entropy oxide according to the invention.

[0072] In some embodiments, the electrochemical cell is included in a lithium-ion battery.

[0073] In another aspect, a high entropy oxide is provided, comprising different metal cations on four or more elements, each metal cation accounting for at least 5% of the total number of different metal cations on the four or more elements in the high entropy oxide, wherein the high entropy oxide comprises a rod-like particle shape.

[0074] High entropy oxides can contain five elementally different metal cations.

[0075] Each metal cation in the high entropy oxide can account for at least 10%, at least 15%, at least 20%, or at least 25% of the total number of metal cations in the high entropy oxide. Each metal cation in the high entropy oxide can account for 5% to 30% of the total number of metal cations in the high entropy oxide.

[0076] Each metal cation may be present in substantially equimolar amounts.

[0077] Each metal cation can be independently selected from the group consisting of cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb and Pt.

[0078] The high entropy oxide may be characterized by one or more of the following: a particle size D50 of about 800 nm or less; and a particle size D90 of about 1200 nm or less.

[0079] The high entropy oxide may be characterized by a particle size distribution having a standard deviation of less than about 250 nm.

[0080] High entropy oxides may include v Co w Ni x Cu y Zn z )O or (Mg v Mn w Fe x Co yNi z )O represented by, wherein v, w, x, y and z are between 0.05 and 0.30. In some embodiments, v, w, x, y and z may be each between 0.05 and 0.30, between 0.1 and 0.3, between 0.15 and 0.25, or about 0.2.

[0081] The high entropy oxide may be characterized in that an X-ray powder diffraction spectrum includes (111) and (200) peaks, wherein the relative peak intensity of I(111) / I(200) is greater than about 1.

[0082] The high entropy oxide may be characterized in that an X-ray powder diffraction spectrum includes (111), (200) and (220) peaks and a relative peak intensity of I(220) / I(200) of about 0.3 to about 1.1.

[0083] The high entropy oxide can be characterized in that the X-ray powder diffraction spectrum includes (111), (200), (220), (311) and (222) peaks, and the relative peak intensity of I(311) / I(222) is less than about 1.

[0084] The length:width ratio of the submicron particles of the high entropy oxide may be between about 1:1.5 and about 1:3.5.

[0085] In another aspect, a method for preparing a high entropy oxide is provided, the method comprising (a) mixing a solution comprising at least four different metal cations on elements in a solvent with a precipitant to obtain a solid material comprising the at least four metal cations, each metal cation accounting for at least 5% of the total number of different metal cations on the four or more elements, and (b) heat treating the solid material to obtain the high entropy oxide; wherein the precipitant comprises an organic anion.

[0086] The thermal treatment may include a calcination process to produce a high entropy oxide intermediate.

[0087] The heat treatment may also include the use of a controlled atmosphere containing a controlled amount of oxygen.

[0088] The method may include annealing the high entropy oxide intermediate to obtain the high entropy oxide.

[0089] The method may include mixing the high entropy oxide intermediate with a solid dispersant prior to annealing.

[0090] The high entropy oxide can be quenched after annealing. The high entropy oxide can be quenched by rapidly cooling the high entropy oxide immediately after the annealing process. Rapid cooling can be achieved by removing the high entropy oxide from the oven and cooling it by exposing it to ambient (e.g., room temperature) air. Rapid cooling can also be achieved by contacting the high entropy oxide with a cooling fluid such as cooled air or liquid nitrogen.

[0091] The solution may contain at least five elementally different metal cations.

[0092] Each metal cation can be independently selected from the group consisting of cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb and Pt. For example, each metal cation can be independently selected from the group consisting of cations of Mg, Co, Ni, Cu and Zn, or independently selected from the group consisting of cations of Mg, Mn, Fe, Co and Ni.

[0093] The precipitant may comprise an oxalate anion. For example, the precipitant may comprise ammonium oxalate.

[0094] The solvent may include water, a combination of water and an organic solvent, or an organic solvent.The solvent may include water and ethylene glycol, such as water and ethylene glycol in a ratio ranging from 1.5:2 to 0.5:2 (by volume).

[0095] In another aspect, a method for preparing a high entropy oxide is provided, the method comprising:

[0096] a. mixing a solution comprising at least four different metal cations on an element in a solvent with a precipitant to obtain a solid material comprising the at least four metal cations, each metal cation accounting for at least 5% of the total number of different metal cations on the four or more elements;

[0097] b. thermally treating the solid material to obtain a high entropy oxide intermediate;

[0098] c. mixing the high entropy oxide intermediate with a solid dispersant and annealing the high entropy oxide intermediate to form the high entropy oxide.

[0099] In another aspect, there is provided an oxalate salt comprising four or more elementally different metal cations, each metal cation comprising at least 5% of the total number of metal cations.

[0100] The oxalate salt may comprise or consist essentially of particles, each of the particles comprising four or more elementally different metal cations.

[0101] Each metal cation can be independently selected from the group consisting of cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb and Pt. For example, each metal cation can be independently selected from the group consisting of cations of Mg, Co, Ni, Cu and Zn, or independently selected from the group consisting of cations of Mg, Mn, Fe, Co and Ni.

[0102] The oxalate can be a solid material with a submicron particle size. The oxalate can have a rod-like shape. For example, the length:width ratio of the oxalate particles can be between about 1:1.5 and about 1:3.5.

[0103] Oxalate can be represented by the formula (A v B w C x D y E z )C 2 O 4 In some embodiments, v, w, x, y, and z are each independently about 0.05 to about 0.30, and A, B, C, D, and E are each independently selected from the group consisting of cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb, and Pt. In some embodiments, v, w, x, y, and z may each be between 0.05 and 0.30, between 0.1 and 0.3, between 0.15 and 0.25, or about 0.2.

[0104] In another aspect, a method for preparing the above-mentioned oxalate is provided, which includes the step of combining a first solution containing four or more elementally different metal cations with a stoichiometric amount of oxalate anions, each metal cation constituting at least 5% of the total number of metal cations in the solution.

[0105] The four or more elementally different metal cations may be present in each oxalate particle in their stoichiometric proportions.

[0106] In another aspect, there is provided a compound having the formula (Mg v Mn w Fe x Co y Ni z )O, where v, w, x, y and z can each be between 0.05 and 0.30, between 0.1 and 0.3, between 0.15 and 0.25, or about 0.2.

[0107] The present invention can also be broadly described as including the parts, elements and features mentioned or indicated individually or collectively in the description of the present application, as well as any or all combinations of any two or more of the parts, elements or features, and in the case where specific integers mentioned herein have known equivalents in the art to which the present invention relates, such known equivalents are deemed to be incorporated herein as if individually set out.

[0108] In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

[0109] As used herein, "(s)" following a noun is intended to refer to the plural and / or singular forms of the noun.

[0110] As used herein, the term "and / or" means "and" or "or", or both.

[0111] As used in this specification, the term "comprising" means "consisting at least in part of..." When interpreting each statement in this specification containing the term "comprising", there may also be features other than the one or those features prefixed by the term. Related terms such as "comprise / comprises" are to be interpreted in the same manner.

[0112] Reference to numerical ranges disclosed herein (e.g., 1 to 10) is intended to also include reference to all rational numbers within that range (e.g., 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9, and 10), as well as any range of rational numbers within that range (e.g., 2 to 8, 1.5 to 5.5, and 3.1 to 4.7), and thus, all subranges of all ranges explicitly disclosed herein are hereby expressly disclosed. These are merely examples of what is specifically intended, and all possible combinations of numerical values ​​between the lowest and highest values ​​recited are to be considered to be expressly stated in this application in a similar manner.

[0113] Although the present invention is broad as defined above, it will be appreciated by a person skilled in the art that the invention is not limited thereto and that the invention also comprises embodiments of which the following description gives examples.

[0114] Other aspects, novel features and advantages of the present disclosure will be apparent to those skilled in the art from any one or more of the illustrative embodiments described in the detailed description and the accompanying drawings. The description and the accompanying drawings should be considered illustrative rather than restrictive in nature. Modifications or improvements may be made without departing from the spirit or scope of the present disclosure and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0115] Several example embodiments will now be described by way of example with reference to the accompanying drawings, in which:

[0116] Figure 1 Shown are (a) a SEM image of the precipitated oxalate precursor described in Example 1, and (b) a TEM bright field image of several oxalate precursor bundles described in Example 1.

[0117] Figure 2 Shown are a) a high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the oxalate beam described in Example 1, with a scale bar of 200 nm, and (b)-(f) EDS elemental mappings of Cu, Co, Ni, Zn and Mg, respectively.

[0118] Figure 3 Thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) and derivative thermogravimetric analysis (DTG, dashed line) graphs of the oxalate precursor described in Example 1 are shown.

[0119] Figure 4 XRD patterns of high entropy oxides annealed from room temperature to different endpoint temperatures and then held at each temperature for 3 hours as described in Example 2 are shown. From bottom to top: 700°C, 800°C, 900°C, and 1000°C. Squares, asterisks, triangles, and hash marks indicate rock salt, tenorite (CuO), spinel (Co 3 O 4 ), wurtzite (ZnO) phase.

[0120] Figure 5 The high entropy oxide (Mg) prepared by solid dispersant assisted annealing is shown. 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O. The projections of different crystal planes are shown next to their corresponding diffraction peaks.

[0121] Figure 6 Rietveld refinement of the corresponding XRD pattern is shown.

[0122] Figure 7 Shown are (a) SEM image of high entropy oxide rods, (b) bright field TEM image of high entropy oxide rods, and (c) high resolution TEM image of high entropy oxide rods, showing rock salt lattice fringes parallel to the (111) plane of the longitudinal axis of the rods, and the inset in the upper left corner is the corresponding FFT image.

[0123] Figure 8Shown are (ac) HAADF-STEM images, ABF images, and contrast-inverted images of the ABF images taken along the

[100] orientation (metal cations and oxygen anions are depicted as large and small spheres, respectively), and (d) ABF image taken along the

[110] orientation (the drift of the O atomic column is shown as an arrow, and the direction of the arrow indicates the drift orientation).

[0124] Fig. 9 Shown are (a) CV curves in the first 5 cycles, (b) CV curves at a crescent scan rate from 0.3 mV / s to 1.0 mV / s, and (c) voltage curves at a current rate of 0.2 A / g in different cycles.

[0125] Fig.10 The cycling performance of the high entropy oxide anode at 0.2 A / g over 470 cycles is shown.

[0126] Fig.11 Shown are (a) rate performance, (b) voltage curves at different current rates, and (c) cycling performance of conventionally annealed (CA) high entropy oxide anode at 0.1·A / g.

[0127] Fig.12 The first discharge curves of high entropy oxide anodes prepared by conventional annealing (dashed line) and dispersant-assisted annealing (solid line) at a current rate of 0.1 A / g are shown, and the inset is the differential capacitance plot (dQ / dV) of the two discharge curves.

[0128] Fig.13 XRD patterns of (a) the product annealed without external oxygen, (b) the product annealed with excess oxygen, (c) the product annealed with external oxygen but insufficient annealing time, and (d) the product according to Example 6 with sufficient annealing and external oxygen are shown.

[0129] Fig.14 An SEM image of the precipitated oxalate precursor described in Example 5 is shown.

[0130] Fig.15 The oxidation states of Mn, Fe, Co, and Ni in the high entropy oxide of Example 6 studied by X-ray absorption near edge structure (XANES) are shown.

[0131] Fig.16 An aspect of the subject matter according to one embodiment is shown.

[0132] Fig.17Shown are a) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the high entropy oxide particles described in Example 6 (scale bar 100 nm), and (b)-(f) EDS elemental mapping of Mg (b), Mn (c), Fe (d), Co (e) and Ni (f).

[0133] Fig.18 A schematic diagram of a half-cell unit described in Example 3 is shown.

[0134] Fig.19 A schematic diagram of a button cell battery described in Example 4 is shown. DETAILED DESCRIPTION

[0135] Examples

[0136] Characterization methods

[0137] The composition of the sample is determined by ICP emission spectrometry (ICP-OES Agilent 5110). At 200 ° C, about 10 mg of the corresponding sample is dissolved in 10 ml of aqua regia in a PTFE beaker. Five different calibration solutions are used for analysis. The structure and phase purity of different samples are characterized by X-ray diffraction (X'Pert Pro MPD with Cu Kα radiation). Thermal analysis is carried out in a Pt crucible in flowing air at a ramp rate of 5 ° C / min on a synchronous thermal analyzer (Netzsch STA449F3 Jupiter). X-ray photoelectron spectroscopy is measured on a ThermoEscalab 250XI with a monochromatic Al Kα source and a 400 μm spot size. All spectra are calibrated with the C1s peak of foreign hydrocarbons at 284.8 eV before fitting.

[0138] SEM images were taken on a field emission scanning electron microscope (Thermo Fisher Scientific Apreo S) operated at 30 kV and 0.4 nA. Bright field TEM images and EDS linear scan results were obtained on a transmission electron microscope (FEI Tecnai F30) equipped with an XFlash 6T-60 EDS detector (Bruker). On an aberration-corrected S / TEM (FEI TitanCubed Themis G2 300, FEI), at an accelerating voltage of 300 kV, the atomic scale characterization of a single high entropy oxide nanorod was carried out with a convergence half angle of 25 mrad. Scanning / TEM was equipped with a monochromator, an EDS detector (Bruker), a Gatan imaging filter (GIF Quantum ER / 965, Gatan) and a high-speed K2 camera (Gatan) of a high-resolution electron energy loss spectrometer. Multiple inelastic scattering backgrounds in the core loss region were removed by Fourier ratio deconvolution of low energy consumption signals. Spectral profiles were collected from the EDS maps of each elemental cation using Gatan DigitalMicrograph GMS3 software. The profiles were converted to text files via a script created by Dave Mitchell (output profile as Tabbed text) and available at the following website: www.dmscripting.com.

[0139] All chemicals were purchased from Sigma Aldrich and used without further purification.

[0140] Example 1a: (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 Coprecipitation of oxalate precursors of )O high entropy oxides

[0141] For the production of high entropy oxides (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 A preferred method of using oxalate anions as precursors of )O) includes the use of oxalate anions to achieve near equimolar deposition of different cations on the elements by forming corresponding complexes in solution. Oxalate anions have been found to form precipitate complexes with Mg, Co, Ni, Cu and Zn cations in polar and protic solutions. Oxalate anions are particularly preferred because they have been found to have similar precipitate formation rates for each metal cation, so that each precipitate particle contains a ratio of metal cations that is substantially equal to the ratio of metal cations in the reaction solution.

[0142] MgCl 2 6H 2 O (99%, 0.55 mmol), CuCl 2 6H 2 O (99%, 0.5 mmol), CoCl 2 6H 2 O (99%, 0.5 mmol), NiCl 2 6H 2 O (99.9%, 0.5 mmol) and Zn(NO 3 ) 2 6H 2 O (98%, 0.5 mmol) was dissolved in a mixture of 10 ml of deionized water and 20 ml of ethylene glycol, labeled solution A. Zinc nitrate is preferred as the zinc source. Zinc chloride is less desirable as a zinc source because it forms insoluble zinc oxychloride in aqueous solution. Excess magnesium chloride was added because magnesium oxalate precipitates are slightly soluble. Then, ammonium oxalate monohydrate ((NH 4 ) 2 C 2 O 4 ·H 2 O, 99%, 2.55mmol) was dissolved in another mixed solution of 10ml deionized water and 20ml ethylene glycol, labeled solution B. Ammonium oxalate is a preferred precipitant because it is soluble in water: ethylene glycol solvent. When oxalic acid is used, the precipitation of metal cations is less uniform compared to oxalates (without being bound by theory, this may be due to the acidity of oxalic acid affecting the relative solubility of metal cations in solution). Group I counterions such as sodium oxalate are insoluble in ethylene glycol, and oxalic acid can cause the precipitated oxalates to dissolve. Both solution A and solution B are heated to 50°C under stirring. Then, solution B (oxalate ions) is quickly poured into solution A (metal ions) under vigorous stirring. The suspension is further stirred at 50°C for 8 hours, and the oxalate precursor is subsequently separated from the reaction solution by centrifugation. The precursor is washed several times with water and anhydrous ethanol and then dried overnight at 70°C.

[0143] Without wishing to be bound by theory, the inventors believe that the complex is formed in a stepwise polymerization. The copper-oxalate complex is formed first, followed by the formation of the other three transition metal ions (i.e., Ni 2+ 、Zn 2+ and Co 2+ ) complexes. Due to the low value of the critical stability constant (log K), Mg 2+ The cation is the most difficult to coordinate with the oxalate ion. Therefore, a chain of magnesium-oxalate complexes is formed in the end. The method provides a precipitated oxalate precursor having a rod-like shape and a layered bundle structure.

[0144] Figure 1 (a) shows a scanning electron microscope (SEM) image of the prepared oxalate precursor. The precursor precipitated according to the above method shows a bundle-like structure. Figure 1 The transmission electron microscopy (TEM) image in (b) demonstrates that the oxalate bundles are monodisperse. The length of each bundle ranges from 500 nm to 1 μm, with an average cross-section of 180 × 180 nm. 2 . TEM images also revealed that the precipitated precursor was dense and solid. Energy dispersive X-ray spectroscopy (EDS) analysis was performed on one of these bundles. The signal of Cu was stronger at the center. In contrast, the signal of Mg was relatively strong at the periphery of the bundle. The signals of Ni, Zn, and Co were almost uniform across the entire oxalate bundle. Therefore, the results of the EDS analysis confirmed the above hypothesis about the layered structure.

[0145] Figure 2 a shows a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of an oxalate beam. Figure 2 In (b)-(f), elemental mappings of Cu, Co, Ni, Zn, and Mg are shown, respectively. It appears that the overall distribution of the five metal cations on the bundle indicates successful coprecipitation of the multicomponent system at the submicron scale. In addition, close observations indicate that the Mg concentration is higher at the periphery, while the Cu concentration is higher in the core (see Figure 2 (b) and Figure 2 (f)).

[0146] The chemical composition of the oxalate precursor was further determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The results indicated that the molar percentage of each metal cation was relatively close to 20%, ie, they were close to equimolar.

[0147] Example 1b: Alternative Anions

[0148] Several alternative anions for preparing high entropy oxide precursor materials have been studied. These anions include hydroxide anions, formate anions, acetate anions and citrate anions. The method of Example 1a is followed, except that oxalate is replaced with stoichiometric equivalents of sodium hydroxide, ammonium hydroxide, hexamethylenetetramine, formate, acetate and citrate. The comments on the suitability of these anions are provided in Table 1 below.

[0149] Table 1

[0150]

[0151]

[0152] Example 2: (Mg from oxalate precursor0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O calcination and high temperature annealing

[0153] To prepare the oxide intermediate, the dried oxalate precursor was calcined in a muffle furnace at 400 °C for 3 h at a ramp rate of 10 °C / min.

[0154] The calcined oxide intermediates were treated using two different annealing methods to form high entropy oxides.

[0155] In a conventional method, the oxide intermediate (calcined, presented as black powder) was annealed in a muffle furnace at different temperatures for 3 hours. The temperatures used were 700° C., 800° C., 900° C. and 1000° C. The ramp rate for all processes was 10° C. / min.

[0156] The second method is a solid dispersant-assisted annealing process. Under stirring, 0.2 g of the oxide intermediate is mixed with 2 g of K 2 SO 4 Redispersed in 40 ml of deionized water for 30 minutes. After ultrasonic treatment for 20 minutes, the suspension was placed in an oven and heated to 120°C. After complete removal of water, the solid product was finely ground in a mortar. The powder was then placed in a ceramic crucible and annealed at 1000°C for 3 hours at the same ramp rate as previously used. After annealing, K 2 SO 4 Dissolved in water, the solid product was isolated via vacuum filtration. After washing the product with sufficient deionized water and drying the sample in an oven, high entropy oxide particles with a rod-like shape were obtained.

[0157] The high entropy oxide can be quenched after annealing. The high entropy oxide can be quenched by rapidly cooling the high entropy oxide immediately after the annealing process. Rapid cooling can be achieved by removing the high entropy oxide from the oven and cooling it by exposing it to ambient (e.g., room temperature) air. Rapid cooling can also be achieved by contacting the high entropy oxide with a cooling fluid such as cooled air or liquid nitrogen.

[0158] Excellent particle size distribution and uniformity as well as excellent electrochemical performance were obtained through an annealing process of at least 5 h.

[0159] Thermogravimetric (TG) analysis of the high entropy oxide from 30°C to 1000°C at a ramp rate of 5°C / min in air showed two significant weight loss stages between 100°C and 400°C ( Figure 3). In derivative thermogravimetric analysis (DTG), the weight loss at lower temperatures, represented by the inflection point at 169°C, corresponds to water loss. The weight loss at higher temperatures corresponds to the decomposition of the oxalate precursor, which is represented by the inflection point in DTG at about 326°C. At 1000°C, about 38% of the mass remains as high-entropy products. Differential scanning calorimetry (DSC) further confirmed that the water loss process is endothermic, while the decomposition of oxalate is exothermic. In addition, a huge endothermic peak located at 740°C was observed. This peak indicates an entropy-driven solid solution process, including Zn 2+ Incorporated into the rock salt structure and spinel Co 3 O 4 It is believed that the mixing of various components at submicron or even nanometer scale in the oxalate precursor facilitates the solid solution process at a lower annealing temperature than the known methods. Figure 4 In the X-ray diffraction (XRD) pattern in Figure 1, Bragg peaks labeled as tenonite CuO can be observed after heating the precursor at 800°C for 3 hours. These peaks disappear after further increasing the temperature to above 900°C, which is consistent with the subtle endothermic peak centered at 830°C in the DSC curve as tenonite CuO is gradually incorporated into the rock salt structure.

[0160] Conventional annealing treatment at 1000°C may result in severe aggregation of the particles. Therefore, in order to avoid the formation of large aggregates, a solid dispersant is used during the high temperature annealing. Without wishing to be bound by theory, it is believed that the solid dispersant inhibits aggregation and crystallite growth. Considering that the annealing temperature can be as high as 1000°C, potassium sulfate is selected as a dispersant because of its melting point of 1069°C. Specifically, the oxalate precursor is first annealed at 400°C for 3 hours to convert the oxalate into a mixed oxide intermediate. The phases in the intermediate are confirmed by XRD. Despite the poor crystallinity, the phases in the intermediate can be labeled as rock salt NiO, black copper ore CuO, rock salt MgO, spinel Co 3 O 4 and wurtzite ZnO. SEM and TEM images reveal that the bundle-like structure is maintained after moderate temperature annealing. Interestingly, the intermediate rods have a mesoporous structure. This is because the thermal decomposition of the oxalate precursor leaves a large amount of internal voids in these rods. The annealed intermediates are finely dispersed in K 2 SO 4 The mixture was then further annealed at 1000 °C for 3 h.

[0161] Figure 5 The XRD patterns in the figure show that (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2)O high entropy oxide is a single-phase compound without any impurity phase. The peak positions are shown in Table 2 below. Figure 6 The Rietveld refinement results in show good convergence and low R-factor, confirming that the (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O high entropy oxide has an fcc cubic crystal structure with The refined lattice parameters are α=β=γ=90°, In this structure, oxygen anions occupy the 4a site, while the octahedral 4b site is randomly co-occupied by Co, Cu, Ni, Zn, and Mg ions with a coordination number of 6. This is a specific example of a high entropy oxide, which is an entropy-stabilized oxide.

[0162] Table 2

[0163]

[0164] Figure 7 (a) shows the SEM image of the annealed HEO rods, confirming that the uniformity and bundle structure of the oxalate precursor are largely maintained after annealing. Unlike the porous intermediate, the HEO nanorods are quite dense, as shown in the bright field TEM image ( Figure 7 (b)) is proved. Figure 7 As shown in (c), the high-resolution TEM image shows the (111) lattice fringes of the high-entropy oxide with a lattice spacing of 0.246nm. The axial (longitudinal) direction of the nanosized rods is along

[111] . The upper left corner represents the corresponding fast Fourier transform (FFT) graph. When compared with the linear scan results of the oxalate precursor, the EDS linear scan across the width of the high-entropy oxide rod shows that the distribution of the five cations in the high-entropy oxide becomes more uniform on the rod.

[0165] Figure 8 (a) shows an atomically resolved HAADF-STEM image of a high entropy oxide nanorod projected along the

[100] orientation. The image clearly reveals that the high entropy oxide has an fcc sublattice of metal cations, in which the oxygen anions are located at the octahedral holes, indicated by four large spheres (Me) and 12 small spheres (O). Figure 8 (b) shows an annular bright field (ABF) image, indicating that the atomic columns of the metal and the O atomic columns are aligned along the

[100] orientation axis. The ABF image along the

[110] direction ( Figure 8(d) shows that some O anions exhibit a slight drift from their perfect octahedral sites. This drift is believed to be caused by the tetrahedrally coordinated Cu 2+ This is caused by the anion sublattice distortion caused by the Jahn-Teller effect.

[0166] Example 3: (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O Electrochemical Performance of High Entropy Oxides The obtained high entropy oxides were used to assemble an electrochemical half-cell unit, in which a lithium disk was used as both the counter electrode and the reference electrode.

[0167] Fig.18 A schematic diagram of a half-cell in the form of a button cell 1816 is shown, which includes a top cover 1802, a high entropy oxide working electrode (anode layer 1804), a polymer membrane 1806, a counter electrode (lithium disk 1808), a stainless steel spacer 1810, an o-ring 1812 and a bottom cover 1814.

[0168] The anode layer 1804 is prepared via a typical slurry method. The high entropy oxide powder synthesized according to Example 2 is mixed with carbon black and PVDF binder to form a slurry in N-methyl-2-pyrrolidone (NMP) in a mass ratio of 8:1:1. The obtained slurry is coated on a copper foil. The film is then heated on a hot plate at 80°C to evaporate the NMP, and then the film is completely dried under vacuum at 80°C overnight. The CR2032 button cell unit is assembled in a glove box under a pure argon atmosphere. Fig.18 In the coin cell 2-electrode configuration shown in FIG, the lithium disk 1808 is separated from the anode layer 1804 by a polymer separator 1806. The electrolyte (not shown) is 1M LiPF dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. 6 The mass loading of active material on the anode layer 1804 is 0.924-1.052 mg / cm 2 .

[0169] Fig. 9 (a) shows a peak at 0.1 mV·s -1 Cyclic voltammetry (CV) curves of the battery cell at a scan rate of . These curves are similar to those of previously reported high entropy oxides (Sarkar et al.). After the first cycle, the strong cathodic peak at 0.52 V decreases significantly, indicating the formation of a solid electrolyte interface (SEI) and the initial reduction of the transition metal oxide to metal and Li 2 O. The subtle cathodic peak at about 1.2 V in the first lithiation can be attributed to Cu2+ / Cu + In the subsequent cycles, four redox peaks can be detected from the CV results. A pair of strong redox peaks centered at 1.2 V (cathode) and 1.8 V (anode) can be attributed to the reduction of transition metal oxides and the reoxidation of metals. In addition, a pair of small redox peaks located at 0.375 V and 0.80 V may originate from the combined effect of the alloying and dealloying processes of Zn metal with Li and the spin-polarized surface capacitance of Co and Ni nanoparticles.

[0170] In previous studies (Sarkar et al. and Ghigna, P.; Airoldi, L.; Fracchia, M.; Callegari, D.; Anselmi-Tamburini, U.; D'Angelo, P.; Pianta, N.; Ruffo, R.; Cibin, G.; de Souza, D.O; Quartarone, E., Lithiation Mechanism in High-Entropy Oxides as Anode Materials for Li-Ion Batteries: An Operando XAS Study [High entropy oxides as anode materials for Li-ion batteries: Operando XAS study]. ACS Applied Materials & Interfaces [ACS Application Materials and Interfaces] 2020, 12 (45), 50344-50354), multiple additional cathode peaks were often observed during the first lithiation process, indicating multiple separate lithiation reactions of the cations involved. Multiple cathode peaks suggest that known high entropy oxide materials can still retain a small amount of binary or ternary oxides in some microregions. After 2 cycles, the CV curves of subsequent cycles are almost the same, indicating the excellent capacity retention ability of the high-entropy oxide electrode. -1 The CV curves at gradually increasing scan rates are Fig. 9 A similar redox trend is shown in (b), which demonstrates the good electrochemical response to different current rates.

[0171] Fig. 9 (c) Galvanostatic charge / discharge curves showing some representative cycles of the half-cell. During the first cycle, the -1 At a current of 1.3 200 mAh·g, the high entropy oxide electrode delivered 1639 mAh·g -1The mechanism of lithium storage in high entropy oxides is through conversion reactions, and therefore the initial coulombic efficiency of the high entropy oxide anode only reaches 50.4%. Then, the lithiation capacity drops to 650 mAh g at the 30th cycle. -1 , and then gradually increased to 1170 mAh·g at the 400th cycle. -1 It is noteworthy that the voltage curves of the 400th and 470th cycles are almost overlapped, indicating that the capacity of the high entropy oxide anode is stable after 400 cycles. Fig.10 The high entropy oxide anode has impressive cycling characteristics, demonstrating that the high entropy oxide of the present invention is impressively stable. In addition, the high entropy oxide anode exhibits excellent rate performance and impressive capacity retention at gradually increasing current rates ( Fig.11 (a)). More specifically, the high entropy oxide anode delivers high specific capacities of 545, 470, 407, and 308 mAh / g at 0.2, 0.5, 1, and 3 A / g, respectively. The capacity stabilizes at about 510 mAh / g at 0.2 A / g in the post-cycle, indicating the excellent structural stability of the high entropy oxide anode under electrochemical operating conditions. Fig.11 (b) shows the charge / discharge curves of the high entropy oxide anode at different current rates. After high rate cycling at 0.2 A g -1 The discharge curve under Fig.11 (b). It appears to overlap with the discharge curve at 0.2 A / g before high rate cycling from 1.5 to 1 V (black dashed line). Similarly, Fig. 9 In (c), the discharge curves in the same range show little change in capacity upon cycling, suggesting that the conversion reaction process is highly reversible.

[0172] The electrochemical performance of the high entropy oxide material prepared according to the present invention was compared with the corresponding material prepared using conventional annealing. The high entropy oxide particles obtained after conventional annealing were also assembled into half-cells. Fig.11(c) shows the cycling performance of the conventional annealed anode (CA-HEO) at a low current rate of 0.1 A / g. Despite the low current rate, the CA-HEO anode exhibited a sudden capacity decay after 280 cycles. In addition, by comparing the first discharge curves of the two high entropy oxide anodes, although the discharge capacities were similar, the lithiation platform observed in the high entropy oxide rod anode was lower than that of the CA-HEO anode. There is a potential difference of about 46.8 mV between the platforms of these two high entropy oxide electrodes. Without wishing to be bound by theory, it is believed that this lithiation potential difference can be caused by the following two aspects. First, the wide particle size distribution of CA-HEO can produce uneven surface overpotentials and lithiation / delithiation kinetics. Secondly, the lithiation rate depends on the crystallographic direction. The axial direction of the high entropy oxide rods synthesized in this study is <111> , and the sidewall planes of the rods are {110} and {112}. These non-close-packed planes are kinetically favorable for lithiation. Even though the small size inevitably leads to a low initial Coulombic efficiency (i.e., about 50%) caused by the massive SEI formation, the short diffusion path and stable 1D structure enable the high-entropy oxide rod anode to have excellent long-term cycling characteristics and rate performance.

[0173] A comparison of the high entropy oxide prepared according to the present invention and two known high entropy oxide anodes is shown in Table 3. The high entropy oxide anode according to the present invention delivers the most impressive electrochemical performance, with the highest ratio of active materials in the anode. In addition, it is believed that the low initial coulombic efficiency can be effectively overcome by various pre-lithiation methods.

[0174] Table 3

[0175]

[0176] Example 4: Electrochemical Cell

[0177] The button cell 1902 including a high entropy oxide anode is constructed from the high entropy oxide of Example 2. Fig.19 A schematic diagram of a constructed button cell 1902 is shown in FIG. The button cell 1902 includes a top cover 1904, an anode 1906, a separator 1908, a cathode foil 1910, a stainless steel spacer 1912, a wave spring 1914, and a bottom cover 1916. The anode 1906 comprises (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O high entropy oxide and is prepared according to the slurry method described above in Example 3. Cathode foil 1910 comprises LiFePO 4 or LiNiCoMnO 2 .

[0178] Example 5: Coprecipitation of oxalate precursors of (MgMnFeCoNi)O high entropy oxide

[0179] Using a method similar to Example 1, another high entropy oxide ((MgMnFeCoNi)O, (Mg 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 )O) is an oxalate precursor. However, some of the divalent cations involved (i.e., Fe 2+ and Mn 2 + ) are susceptible to oxygen because they are very easily oxidized to their higher valence states. Therefore, the preparation of the precursor is carried out in an inert atmosphere using a Schlenk line so that a pure Ar (argon) environment can be used during the synthesis. By first adding 0.1 mmol of ascorbic acid (C 6 H 8 O 6 ) was dissolved in 15 ml of deionized H 2 The sample was prepared by mixing 100 ml of 1% O and 15 ml of ethylene glycol. Ascorbic acid is a reducing agent that can effectively prevent Fe 2+ and Mn 2+ Then, in a round-bottom flask, 1.1 mmol MgCl 2 (98%), 1mmolMnCl 2 ·4H 2 O(98%), 1mmol FeCl 2 (98%), 1mmol CoCl 2 6H 2 O (99%) and 1mmol NiCl 2 6H 2 O (100%) was dissolved in the above solution. After all the metal chlorides were added to the solution, the flask was quickly connected to a Schlenk line and purged with argon three times. The metal ion solution was heated to 50°C with stirring. In deionized H 2 In another mixed solution of O and ethylene glycol (15ml+15ml), 5.1mmol of ammonium oxalate monohydrate (NH 4 ) 2 C 2 O 4 ·H 2 O. Then, the solution was deoxygenated using a Schlenk line and then injected into the chloride solution under vigorous stirring. After reacting at 50°C for 6 hours, the oxalate precursor was washed and separated via centrifugation several times, and then the precursor was dried at 50°C overnight.

[0180] Fig.14 The oxalate precursor (MgMnFeCoNi)C 2 O 4 SEM image of rod-shaped particles.

[0181] Example 6: Preparation of (MgMnFeCoNi)O high entropy oxide

[0182] The (MgMnFeCoNi)O high entropy oxide was directly formed as a single-phase solid solution by calcining the oxalate precursor at high temperature. The precursor was calcined at 1000°C for 5 hours in a covered corundum ceramic boat in an Ar atmosphere using a tube furnace. The precursor was placed in a quartz pot in the boat, and MnO as an oxygen generator was added. 2 Positioned next to the quartz pot. Using a quartz pot allows MnO 2 The precursor sample was positioned close to the sample while preventing contact and contamination. The amount of precursor powder was 300 mg, and MnO 2 The amount was 90 mg. Maintaining a temperature of 1000° C. for an extended duration of at least 5 hours was found to be an effective annealing process that produced a high entropy oxide with good purity.

[0183] After formation, the high entropy oxide was allowed to cool naturally. The dark brown powder obtained was found to be stable under room temperature ambient conditions as it showed no change in phase purity after exposing the sample to air for several weeks.

[0184] It was found that if the calcination and annealing process was carried out in a pure Ar atmosphere without any oxygen source, the resulting material included a mixture of wüstite (FeO) and Ni alloy ( Fig.13 (a)). The presence of the metallic phase can be attributed to the generation of reducing byproducts (ie, carbon and carbon monoxide) due to the decomposition of the oxalate ligands during the heat treatment.

[0185] MnO 2 Used as an oxygen generator during calcination to mitigate the effects of the reducing environment. MnO 2 It gradually decomposes at high temperatures and releases a small amount of O2, neutralizing the reducing substances or slightly oxidizing the metal products formed. The covered boat provides sufficient sealing to maintain the oxidizing environment. MnO 2 Undergoes thermal decomposition as follows:

[0186] (400℃ to 800℃)2MnO 2 =Mn 2 O 3 + 1 / 2 O 2

[0187] (above 800℃)Mn2 O 3 =2Mn 3 O 4 + 1 / 2 O 2

[0188] Fig.13 (b)-(d) show that after adding MnO 2 XRD patterns of samples formed with external oxygen source. 2 The excess oxygen decomposes to produce spinel products (AB 2 O 4 , A=Mg, Mn, Fe, Co, Ni; B=Fe, Mn), such as Fig.13 (b) shows the formation of spinel ferrite because Fe 2+ and Mn 2+ Ions are MnO 2 The extra oxygen produced by decomposition is oxidized to Fe in proportion 3 + and Mn 3+ . Trivalent Fe 3+ and Mn 3+ With the remaining divalent cations (Mg 2+ , Mn 2+ , Fe 2+ 、Co 2+ , and Ni 2+ ) produces a spinel product. These undesirable components can be avoided and / or minimized by introducing an appropriate amount of oxygen into the annealing process. This amount can be easily determined by simple testing.

[0189] Fig.13 (c) It shows that the controlled addition of MnO 2 The coexistence of spinel and metallic phases in the oxide product calcined at 1000°C and annealed by holding at 1000°C for one hour is shown in Figure 1. Insufficient annealing produces an over-oxidized top layer and an under-oxidized bottom layer due to the limited oxygen diffusion rate in the fixed bed. This problem is solved by extending the annealing duration. Fig.13 (d) shows that Fig.13 Compared with the product in (c), when the same amount of MnO 2 When the calcined oxide was annealed, an annealing time of 5 hours produced a well-crystallized single-phase solid solution with a rock salt crystal structure. The peak positions are shown in Table 4.

[0190] Table 4

[0191]

[0192] The oxidation state of each metal species was studied by X-ray absorption near-edge structure (XANES), and Fig.15 The results in indicate that the four transition metal elements consistently have an average state close to +2. The slight increase in the pre-edge intensity of the FeO reference material can be attributed to the octahedral coordination of Fe 2+ Slightly oxidized to Fe with tetrahedral geometry 3+ .

[0193] Fig.16 SEM images of annealed high entropy oxide materials are shown. The SEM shows rod-shaped particles and some agglomeration between particles. The use of solid dispersants (such as those described herein) will reduce agglomeration. Fig.17 (a) shows dark field scanning transmission electron microscopy (DF-STEM) analysis, and Fig.17 (b)-(f) show energy dispersive spectroscopy (EDS) mapping of high entropy oxide particles. The uniform distribution of different metal species can be clearly observed in the representative (MgMnFeCoNi)O particles.

[0194] It is not intended to limit the scope of the invention to only the examples described above. As will be appreciated by those skilled in the art, many variations are possible without departing from the scope of the invention as set forth in the appended claims.

Claims

1. A method for preparing a high entropy oxide, the method include: (a) mixing a solution comprising at least four elementally different metal cations in a solvent with a precipitant to obtain a solid material comprising the at least four metal cations, each metal cation constituting at least 5% of the total number of the four or more elementally different metal cations; (b) heat treating the solid material to obtain a high entropy oxide; The precipitant comprises an organic anion.

2. The method according to claim 1, in, The thermal treatment includes a calcination process to produce a high entropy oxide intermediate.

3. The method according to claim 2, in, The heat treatment includes annealing the high entropy oxide intermediate to obtain the high entropy oxide.

4. The method according to claim 3, in, The high entropy oxide intermediate is mixed with a solid dispersant prior to annealing.

5. The method according to any one of claims 1 to 4, in, The heat treatment involves the use of a controlled atmosphere.

6. The method according to any one of claims 1 to 5, in, The solution contains at least five elementally different metal cations.

7. The method according to any one of claims 1 to 6, in, Each metal cation is independently selected from the group consisting of cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb and Pt.

8. The method according to any one of claims 1 to 7, in, The metal cations are independently selected from the group consisting of cations of Mg, Co, Ni, Cu and Zn.

9. The method according to any one of claims 1 to 8, in, The metal cations are independently selected from the group consisting of cations of Mg, Mn, Fe, Co and Ni.

10. The method according to any one of claims 1 to 9, in, The precipitating agent is an oxalate compound.

11. The method according to any one of claims 1 to 10, in, The solvents include water and ethylene glycol.

12. The method according to claim 10 or 11, in, The oxalate compound is ammonium oxalate.

13. A method for preparing a high entropy oxide, the method include: (a) mixing a solution comprising at least four elementally different metal cations in a solvent with a precipitant to obtain a solid material comprising the at least four metal cations, each metal cation constituting at least 5% of the total number of the four or more elementally different metal cations; (b) thermally treating the solid material to obtain a high entropy oxide intermediate; (c) mixing the high entropy oxide intermediate with a solid dispersant and annealing the high entropy oxide intermediate to form the high entropy oxide.

14. An oxalate salt comprising four or more elementally different metal cations, each metal cation constituting at least 5% of the total number of metal cations.

15. The oxalate according to claim 14, in, Each metal cation accounts for 5% to 30% of the total number of metal cations.

16. An oxalate salt as claimed in claim 14 or 15 in the form of particles comprising different metal cations on the four or more elements.

17. The oxalate salt according to any one of claims 14 to 16, in, Each metal cation is independently selected from the group consisting of cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb and Pt.

18. The oxalate salt of any one of claims 14 to 17 comprising a rod-like particle shape.

19. The oxalate according to claim 18, in, The length:width ratio of these particles is between about 1:1.5 and about 1:3.

5.

20. The oxalate salt according to any one of claims 14 to 19, which is represented by the formula (A v B w C x D y E z )C 2 O 4 Represents, wherein v, w, x, y and z are each independently from about 0.05 to about 0.30, and wherein A, B, C, D and E are each independently selected from the group consisting of cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Gd, Pb and Pt.

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